Velocity discrepancy

[ke5hvm]

Good morning,

I have been studying over my flight data from two different altimeters from a recent flight, and am seeing two very different velocities, and I wasn't sure which one was generally accepted as being more accurate.

My Jolly Logic Altimeter III is an inertial accelerometer, and displayed an altitude of 6,322 feet and a peak velocity of 642 mph (Mach 0.8).

My Stratologger CF-100 is a data-logging barometric altimeter, which controls my ejection charges. It read an altitude of 6,300 feet (A difference of 22 feet, or 0.34% difference between the Altimeter III...close enough for me).

Both altimeters log velocity and altitude data at a rate of 20 samples per second.

However, the velocity recorded by the Stratologger was 927 mph (Mach 1.2), a considerable variation from the accelerometer-based altimeter.

So, I was wondering which one was generally accepted as the more accurate device for velocity, and why there might be such a discrepancy.

Additionally, when I simmed the flight, I got a predicted altitude of 5,000 feet, which I exceeded by over 1,300 feet, and a simmed velocity of Mach 1.0, so it does seem possible that I could have made it past Mach 1.

Any thoughts or opinions are greatly appreciated.

 

[Exactimator]

Accelerometer based velocity data is more accurate than barometer based. So the Jolly Logic's will be more accurate.

If you have the Perfectflight data connection cord, download the flight data from the altimeter. If you actually got near Mach, the graphs of both altitude and velocity will bounce all over while the rocket's getting close (around Mach 0.8 to about 1.2). This is the result of the Mach shockwave wreaking havoc with the baro sensor. You'll see from the graph that any velocity data in that region is worthless.

As much as it makes the data unreliable, I enjoy seeing the data spikes because it means I went super speedy.

 

[Maxwelljets]

Accelerometer based velocity data is more accurate than barometer based. So the Jolly Logic's will be more accurate.

This statement has an important caveat though. If your rocket's maximum acceleration was higher than the accelerometer can handle, it maxes out the accelerometer and the max velocity it calculates will be thus be lower than reality.

 

[Exactimator]

This statement has an important caveat though. If your rocket's maximum acceleration was higher than the accelerometer can handle, it maxes out the accelerometer and the max velocity it calculates will be thus be lower than reality.

Thanks, I forgot about that. I think the JL max acceleration along one axis is somewhere around 23 Gs.

 

[Cameron Anderson]

Accelerometer based velocity data is more accurate than barometer based. So the Jolly Logic's will be more accurate.

If you have the Perfectflight data connection cord, download the flight data from the altimeter. If you actually got near Mach, the graphs of both altitude and velocity will bounce all over while the rocket's getting close (around Mach 0.8 to about 1.2). This is the result of the Mach shockwave wreaking havoc with the baro sensor. You'll see from the graph that any velocity data in that region is worthless.

As much as it makes the data unreliable, I enjoy seeing the data spikes because it means I went super speedy.

Is this really the case though? Maybe 10 years ago before there was better mach immune filtering on altimeters this might have been the case but I ignore accelerometer altitude and just look at baro.

I use multiple devices on my flights so I always have two sets of data to compare. Baro altitudes are always within 1-2% of each other which are both within 1-2% of my GPS altitude and my accelerometer data is always the outlier and is significantly higher.

 

[Maxwelljets]

Is this really the case though? Maybe 10 years ago before there was better mach immune filtering on altimeters this might have been the case but I ignore accelerometer altitude and just look at baro.

I use multiple devices on my flights so I always have two sets of data to compare. Baro altitudes are always within 1-2% of each other which are both within 1-2% of my GPS altitude and my accelerometer data is always the outlier and is significantly higher.

Yes, that statement is still true. All the filtering does is prevent deployment due to pressure spikes, it does nothing to actually increase the accuracy when the measured pressure differs from the true pressure (which almost always happens when the rocket is transonic). Barometric peak altitude is going to be quite accurate, but the max velocity as measured by a barometric altimeter is going to be quite inaccurate compared to one derived from accelerometer data.

 

[Voyager1]

.....
Additionally, when I simmed the flight, I got a predicted altitude of 5,000 feet, which I exceeded by over 1,300 feet, and a simmed velocity of Mach 1.0, so it does seem possible that I could have made it past Mach 1.

Any thoughts or opinions are greatly appreciated.

How close to reality was your simulation model? That is a huge difference! Had you accounted for the actual launch mass? Rocket body surface finish? Fin profile? Wind? Launch angle?

Additionally, were the two altimeters co-located?

 

[manixFan]

Your best solution is to import the data into a spreadsheet and manually compute the velocity and graph it. You'll be able to see if the reported velocities are sensible or not. I remember one flyer who thought he hit Mach 2+ because one sample went to that velocity, but when graphed it was clear it was a spurious sample. You can also smooth the data by averaging it out over several samples. It's really the best way to see what's going on with your data. When I have a sample that is way out of whack, I replace it with the average of the sample before and after.


Tony

 

[Exactimator]

Is this really the case though? Maybe 10 years ago before there was better mach immune filtering on altimeters this might have been the case but I ignore accelerometer altitude and just look at baro.

I use multiple devices on my flights so I always have two sets of data to compare. Baro altitudes are always within 1-2% of each other which are both within 1-2% of my GPS altitude and my accelerometer data is always the outlier and is significantly higher.

Thread title refers to velocity. My previous post notes "Accelerometer based velocity data is more accurate than barometer based." I trust accelerometer for velocity, baro for altitude. YMMV.

Altimeters don't filter. They include a time delay that provides Mach lock-out protection.

Here's a graph from the Stratologger CF in my L3 cert flight, max velocity Mach 1.2. Note the red velocity curve at the left side of the graph where it's transitioning Mach. It's garbage.


Here's a similar graph of a flight to Mach 1.5. Again the velocity trace is garbage. If you look closely, you can see the altitude graph bounce in the same time period.


All of my Mach flights have a velocity and altitude trace that looks like those above. Now the altitude I trust because the air pressure smooths out again when the rocket drops below Mach, well before apogee.

Non-Mach flight graphs look like this:

 

[ke5hvm]

Thank you all for your replies.
Looking at my data from the barometric altimeter (Stratologger CF), I have a nice smooth velocity curve.
From what physics I remember, as you approach Mach 1, the drag increases, and the energy required to break the speed of sound increases, and you need that extra "oomph" to push through. I was using a K-535, with a burn time of about 2.75 seconds.
My suspicion was that the accelerometer would be the more accurate of the two for velocity measurements. As for the simulation discrepancy, well...I guess I've got some work to do.
I've been using OpenRocket for several flights of this airframe (4" Madcow Frenzy), and my simulations over-predicted the altitude by 12%...consistently. So when it came to flying time, I would take off 12%, go fly and, upon checking both altimeters, they would be spot-on with my corrected value.
The K-535 was the largest motor I've flown, so when I simmed it out, I took off my 12% and was expecting something reasonably close. I was shocked and thrilled when both altimeters recorded a mile+ flight. So, now, I need to figure out why so I can more accurately predict future flights on that motor, now that I have a better idea of how that rocket performs.

Thanks again for your thoughtful responses. They will all help me plan future flights which will hopefully include a Mach 1+ flight soon.

Take care and fly safe.

Sincerely,
Gerrit Rickwalt
TRA L2

 

[Buckeye]

Also, numerical integration of acceleration is more accurate than numerical differentiation of altitude.

The best assessment of velocity comes from neither baro nor acceleration altimeter, but from a good simulation!